Self-threading (thread forming) screws

[bernardg]

Hello,

We manufacture the dc motors. And for attaching the end frames and the shell, we plan to replace the bolt & nut assembly, by self-threading screws (thread forming). This is still in the testing stage. But one of our concerns is that, say while in the application, we need to unscrew it and retighten it.... what implication will that have on the holding power? Will that work as efficiently as we screw them, the first time? or will it loose a significant amount of holding power?

Please clarify this for me.

Thank you very much!
~Bernard

 

[peebee]

Sounds like a question for the Welding, Bonding & Fastener engineering Forum (5424 members)
forum725

 

[Thorn3]

Hello,
Self threading screws have never been a reliable fastener, in my own personal experience, when you wish to be capable of adjustments such as retightening. They are generaly used to fasten items that are not intended to experience high vibration or adjustability of any sort. I have rarely seen, if ever, a self threading screw truly cut a thread cleanly. There is usually a great deal of stress applied to the socket where the screw is driven. If you wish to maintain any form of control over how tight a given joint is, stick with machined threads, either with the use of a thru hole and bolt assembly, or tapped holes. Of course there are many, many variables, such as material, depth of hole etc.

 

[Lewish]

Thorn3 said it very well. Self-threading fasteners are a tighten once and forget them item. Once they loosen, they will break before you can torque them to the previous tightness.

 

[Thorn3]

Hello Mpoc,
Lewish is of course quite correct, luckily, in far less words than mine. What worries me here, from an engineers perspective, is that you appear to be falling into the cold and clammy hands of "the financial demon". It is always the goal of a true engineer, to make his products the very best they can be, to offer the consumer the very best for their money.
If you use self threading screws, first think about the stresses you will be appling to the material you are driving the screw into. Will this compromise the stability of the device overall? More than likely. Will it prevent adjustability in the future? Certainly. There are always good ways to overcome manufacturing costs. However, compromising the overall structural integrity of a structural element, is not one of them. Do not worry, and do not make rash changes. They will prove more costly in the end. The finacial demon is greedy, but is easily beaten by the clever engineer.

 

[jbartos]

Suggestion: If those dc motors are meant to be disposable, perhaps, one could consider powder driven studs or nails.

 

[bernardg]

Thorn & Lewish,

Thanks for your suggestions. But some of our competitors are using the very same procedure and still pass the random vibration testing.

I should say that I am very confused at this point. Will you please guide me to the resources where I could get more information about the thread forming screws?

Thanks a lot,
bernard

 

[Thorn3]

Hello,
Jbartos is correct with his suggestion regarding throw away motors. You indicated that your motors have, in the past, been assembled with the use of thru-bolts, with nuts. This is normaly, from my experience, been done on motors that can be re-opened for such repairs as brush replacement, bearing or armature etc, with the ability to reseal the motor. This indicates to me, that your motors are not originaly designed to be thrown away, but to repair them as needed.
I would first suggest a full analysis of the motors application range. Some motors, in certain applications, require more maintenance than others. If you truly believe that said motors are very low maintenance, if any, then it may be practical to utilize an alternative fastener.
I personaly, do not ever, ever.. see self tapping screws as a viable alternative for a device, any device, that is intended to be disassembled and reassembled.
This is why: When a self tapping thread is forced into a hole, it applies a specific pressure against the holes inner wall. Depending on the material being screwed, there will exist, a certain back-pressure or spring-back of the material against the screw. This is part of the reason that the screw stays tight. If you remove such screws from their hole, a certain amount of friction is going to occure, that will abrade the inner surface of the hole, preventing the screw from ever achieving the same holding power again. The chances of the threads properly realigning when installed the second time, is slight. This will cause further degradation of the hole, thus decreasing holding power.
If you give the specifics as to why you would need adjustability, we could more easily make more direct, to the point suggestions. Many things truly are application dependant. Also, you will find that many engineers are biased one way or the other, regarding self threading fasteners. I myself, do not like them in applications where they will have to be removed, then reinserted. As Lewish stated previously, they are really only good for a one time shot, "tighten once then forget them".
What type/size of motors are these?
What load/vibration are they expected to maintain?
What is the end cap material/thickness?
How many threads, per screw diameter,are actually in the hole?
What is the reason for disassembly/reassembly/adjustability?
How many times is the motor expected to go through being adjusted etc, within its working life?

By the way, I have been in contact with the CEO of this site, to suggest a new button/function for this site, that will enable the use of pictures for everyone. I was told that they like the idea and will attempt to insert the function into the site. This will make everyone's life much easier, as we can actually see what people are talking about.
Along that note, if you are authorized to do so, please send me a drawing or CAD file of the motor as it is now, and the changes you hope to make. This will offer a much better analysis of the situation and we can all stop assuming and truly find a proper solution.
Send any files you have to: thorn21@cableone.net
I presently use AutoCAD2000, which produces a .dwg file format.

 

[Foxfur]

If your company is determined to implement the use of thread forming screws, please make a suggestion on my (and many other end users) behalf.
Where possible, make the hole a through hole (as on an overhanging flange) so that the original hole may be drilled out to the next screw size allowing us in the field to replace one screw with one machine screw and nut, instead of the motor. If that is not possible, perhaps you could suggest that they leave enough web thickness in a mono-body (where the screw hole is truly blind) so that field tech could simply pilot drill to the next size and tap the hole for a larger screw.
I take notice of the servicability of all parts on systems I design and build. If they are not repairable (within reason) then they usually don't make the team.
Thanks,
Ff

 

[bernardg]

Thorn,
I have sent you an email. Thank you very much for your help. I am eager to hear from you.
~bernard.

 

[bernardg]

Ff,
For "sealing and water leak problems", we plan to replace the bolt & nut assembly, by self-threading screws. The self-threading screw is tightened into a blind hole on the casting, which eliminates the two primary leak paths of the motor.

There are lot of advantages for us to go with the self-threading screws. And the bottom line is, there would still be some disassembling of motors, once in a while.

All we are concerned at this point is, how sensible this choice is.

 

[bernardg]

Thorn,

Thank you very much for your detailed step-by-step clarification for all the querries that i had.

I will cover the base by following the Engineer's thumb rule: "Tell a man there are 300 billion stars in the universe and he will believe you. Tell him a bench has wet paint on it and he will have to touch, to be sure"

We are proceeding with the self-threading screws. I'll test 10 motors with different reassembled patterns and do a random vibration test to qualify the self-threading screws.
I'll keep you posted.

Thanks Thorn...
and Thanks to all people who spend time in helping others.

Sincerely,
bernard.

 

[Thorn3]

Hello mpoc,

You are most welcome for any assistance I was able to offer. Always stick to your guns when you believe that what you think or feel is correct. After all, the only true test to success is your own determination to make something work. By the way, your Engineer's rule of thumb is quite funny, and true! Here is another funny one:
I once saw a man, who needed to locate the position of a hole in a piece of sheet metal. He grabed a dial caliper, set it exactly, then used a wide tipped marker to mark the location. Then, he proceeded to drill it with a hand drill. The drill bit walked a bit, placing his hole about .25" off, on two axis! Simply amazing! He was originaly a carpenter, so what is to be expected?
As far as Foxfur indicated: I agree with his thoughts. Then again, it is always the idea of the engineer to make things with the very best concepts and manufacturing processes. This is because the engineer tends to look at things in a whole different way from the average person, or persons from other fields. I personaly would love to have a motor that is more easy to manipulate to my exact applications. His idea of either a thru hole or a tapped hole that can be modified more readily is practical in the field, which is something that many design engineers overlook. After all, many engineers are trained and experienced in designing devices, not necissarily using them in the field.
If your motors are to be sealed, keep in mind that it may be most applicable, to add a rubber washer under the head of the screw/bolt. I realize that only one end is intended to be subjected to moisture, but moisture doesn't always play along the way we wish it to. I hope you will be sealing both ends, just to cover all the unexpected occurances, such as capillary action, condesation, travel of condensed fluids due to vibration and gravity etc.
Foxfur, I understand and agree to your suggestions regarding the manipulation of a hole. I run into this problem all the time. If I run into such a motor that is great for the application, but does not offer such modifications, I make them as soon as they come out of the box. This is not practical by any means, as that means that I have to spend time/money to create the mods. But, then again, I can see it from the manufacturers point of view as well. Even one more manufacturing process, washer, etc, can make or break the finances of creating the motor, and obtaining a suitable profit.
Since I develope prototypes, it is a cross between engineering and manufacturing. So, I see both sides equally. I have yet, in my applications, come across a motor that fit my needs exactly. Frustrating at times, but somewhat unavoidable in low volume production.

mpoc, I would love to see/hear the results from your testing, as you know my initial feelings towards self tapping screws. It is hard to be unbiased, after dealing with design issues in the field. I hope your experiences with this, can show me a thing or two. Feel free to contact me at any time.